1. Field of the Invention
The present invention relates to a piston-type compressor, in which fluid is compressed by means of reciprocating pistons connected to a swash plate. More particularly, it relates to a configuration of reciprocating pistons, which reduces the weight of the pistons in the refrigerant compressor for an automotive air conditioning system.
2. Description of the Related Art
A variable capacity, swash plate-type compressor is disclosed in U.S. Pat. No. 4,664,604, which is incorporated herein by reference. Referring to FIG. 1, a cylinder block 13 is accommodated in cylinder housing 11 of a compressor 10. Pistons 28 are accommodated in cylinder bores 27 and are reciprocatedly moved therein. A drive shaft 15, which is driven by an engine (not shown), is rotatably supported by means of the central portion of cylinder block 13 and a front cover 22. Rotor plate 18 is mounted on drive shaft 15, and synchronously rotates with draft shaft 15. Further, a swash plate 24 is tiltably mounted on the drive shaft 15 and is reciprocally slidable together with a sleeve 30 parallel to the axis of drive shaft 15. Rotor plate 18 and swash plate 24 are connected to each other by means of a hinge mechanism. Swash plate 24 engages the interior portion of an associated piston 28 along its circumference.
According to the above-described compressor, when drive shaft 15 is rotated, rotor plate 18 rotates together with drive shaft 15. The rotation of rotor plate 18 is transferred to swash plate 24 through the hinge mechanism. Rotor plate 18 is rotated with a surface inclined with respect to drive shaft 15, so that pistons 28 reciprocate in cylinder bores 27, respectively. Therefore, refrigerant gas is drawn into an inlet chamber and compressed and discharged from the inlet chamber into an associated discharge chamber.
Control of displacement of this compressor is achieved by varying the stroke of piston 28. The stroke of piston 28 varies depending on the difference between pressures which are acting on the opposing sides of swash plate 24. The difference is generated by balancing the pressure in a crank chamber acting on the rear surface of piston 28. Consequently, such the suction pressure in cylinder bore 27 acting on the front surface of piston 28, which suction pressure acts on swash plate 24 through piston 28.
In the above-mentioned variable capacity, swash plate-type compressor, it is desirable to reduce the load that is applied to the compressor's drive source, e.g., a vehicle engine. To accomplish this, piston 28 is preferably lightweight. Accordingly, a main body of each piston 28 which reciprocates in cylinder bore 27 is formed with an open space 28a therein. A protrusion 29 thereof axially extends from the main body to engage a radial aperture at the periphery of swash plate 24 via sleeve 30.
A second approach to reducing the weight of the pistons is disclosed in unexamined Japanese Utility Patent Publication No. H4-109481, published on Sep. 2, 1992. Referring to FIG. 2, a piston 88, which includes cylindrical body 38a and close hollow portion 38b therein, is produced, such that at least two separated cylindrical hollow elements are joined together by welding. An arm portion 38c extends from cylindrical body 38a.
A third approach to reducing the weight of the pistons is disclosed in unexamined Japanese Utility Patent Publication No. H7-18989 and H7-18900, both published on Jul. 28, 1995. Referring to FIGS. 3a and 3b, a piston 48 has a solid, cylindrical body 48a. A first aperture 48b and a second aperture 48c are formed on the periphery of cylindrical body 48a, such that these apertures communicate with each other. Referring to FIG. 4, a piston 58 has a cylindrical body 58a and a recessed portion 58b formed on a half radial, side surface of cylindrical body 58a. Recessed portion 58b is scooped out toward the interior of cylindrical body 58a of piston 58.
Nevertheless, the pistons discussed above have at least the following disadvantages. In the piston of FIG. 1 described in U.S. Pat. No. 4,664,604, hollow portion 28a of piston 28 does not maintain a large capacity therein because a bite of machine metals is not inserted deep into the interior of piston 28 from one axial end of piston 28 toward the longitudinal axis of piston 58. In the piston of FIG. 2 described in unexamined Japanese Utility Patent Publication No. H4-109481, when the piston is produced by a forging machine, closed hollow portion 38b of piston 38 is formed by scooping out material from one end portion near the piston head toward an arm portion 38c of piston 38. If a welded jointed portion is to be placed near the piston head, cylindrical hollow portion 38b near arm portion 38c has a smaller radial inner diameter than the piston head. Moreover, inner diameter of cylindrical hollow portion 38b gradually decreases toward arm portion 38c because a core inserted into cylindrical hollow portion 38b for forgoing is drawn out from molding die. Thus, an area having a small diameter is added during the cutting process in order to maintain a uniform diameter and to prevent the above-mentioned disadvantages. Accordingly, this configuration results in increasing the overall weight of piston 38 or in increasing the production cost of piston 38, or both.
On the other hand, if a welded jointed portion is placed near arm portion 38c, the frictional force which is generated by the sliding of swash plate 24 within sleeve 30 is transferred to piston 88 and urges piston 38 to rotate around its axis and to include in a radial direction. In particular, because the movement perpendicular to drive shaft 15 and to the longitudinal axis of piston 38 acts on the welded joint portion of cylindrical body 38a, the welded joint portion is easily broken.
In the piston of FIGS. 3a and 3b, described in unexamined Japanese Patent Publication No. H7-189898, and FIG. 4, described in unexamined Japanese Utility Patent Publication No.H7-189900, the radially periphery surface of cylindrical body 48a of piston 48, which makes contact with the inner surface of cylinder bore 27, decreases because apertures 48b and 48c of a recessed portion cover the greater parts of the radial periphery surface of cylindrical body 48a or 58a of piston 48 or of piston 58, respectively. Therefore, gas compressed within cylinder bore 27 leaks out to the crank chamber because the sealing area decreases between the radial periphery surface of piston 48 or of piston 58, and the inner surface of cylinder bore 27. As a result, the efficiency of the compressor is reduced.